1. Field of the Invention
The present invention generally relates to an optical semiconductor device having an optical non-linearity, a light switching apparatus using such an optical semiconductor device, and a light switching method.
Recently, there has been considerable activity in the research and development of optical semiconductor devices having an optical non-linearity in order to realize light-light switches, optically bistable devices, light-light memories and so on. Such an optical semiconductor device has a nature resulting from the optical non-linearity in which excitons and free electron/hole pairs are excited and the light absorbing coefficient and refractive index are thus varied. However, the currently proposed optical semiconductor devices have problems to be solved.
2. Description of the Prior Art
FIG. 1 shows a light-light switching apparatus and a light switching method utilizing an optical non-linearity dependent on spins. The apparatus and method shown in FIG. 1 is disclosed in T. Kawazoe et al., "HIGHLY REPETITIVE PICOSECOND POLARIZATION SWITCHING IN TYPE-II AlGaAs/AlAs MULTIPLE QUANTUM WELL STRUCTURES", Jpn. J. Appl. Phys. Vol. 32, 1993, pp. L1756. A linearly polarized light 26 is split into two light components by means of a beam splitter (BS) 28. One of the two light components is a control light 30, and the other light component is a signal light 32. A quarter wave plate 34 is provided on the optical axis of the control light 30 from the beam splitter 28. The quarter wave plate 34 converts the incident light into a right circularly polarized light. That is, the control light passing through the quarter wave plate 34 is a light-handed circularly polarized light. This light enters a mirror pair (MP) 36, which forms a light train of 80 GHz. The light train from the mirror pair 36 is converged by a lens (L) 38, and is projected onto an optical semiconductor device (S) 40 including an optically non-linear element.
The signal light 32 outgoing from the beam splitter 28 passes through the lens 38 while the linear polarization is maintained, and is then projected onto the optical semiconductor device 40. In order to facilitate the better understanding of the following description, it will now be assumed that the signal light 32 is a longitudinally polarized light (S-polarized light).
The longitudinally polarized light can be divided into a right circularly polarized light component and a left circularly polarized light component. When the signal light 32 is projected onto the optical semiconductor device 40 in a state in which the control light 30 having the right circular polarization, the absorption coefficient for the right circularly polarized light component is reduced due to the influence of the control light 30, while the left circularly polarized light component is not influenced. In this way, light switching is implemented.
The signal light 32 passes through the quarter wave plate 42, which converts the right circularly polarized light component into a transversely polarized light (P-polarized light) and converts the left circularly polarized light component into a longitudinally polarized light. A Wollaston prism (WP) 44 separates the transversely polarized light and the longitudinally polarized light from each other. These separated lights are respectively detected by photodiodes (PD) 46. Then, the difference between the outputs of the photodiodes 46 is produced, so that light switching can be observed. The spins are relaxed with a few picoseconds to tens of picoseconds, so that the up spins and down spins are balanced. As a result, the difference between the signals becomes zero after spin relaxation.
As described above, the signals are recovered at an extremely high speed, so that an optical semiconductor device capable of performing high-speed repetitive operation can be realized.
However, optical semiconductor devices as described above have the following disadvantage. The switching operation utilizes a variation in light absorption caused by projecting of the control light. Hence, the intensity of output signals from the photodetectors is very weak.
There is another disadvantage. A logical operation is impossible in which a plurality of control lights are given to the optical semiconductor device. In other words, the switching operation cannot obtained unless both the control light and the signal light are incident to the optical semiconductor device. That is, only the AND operation can be performed.
Further, the optical switching apparatus shown in FIG. 1 needs processing of electric signals output by the photodiodes 46, and is not a truly optical apparatus.